A fuel cell is a generator for converting chemical energy generated by oxidation of fuel into electrical energy.
The fuel cell includes a plurality of unit cells coupled in series, and the number of unit cells coupled in series ranges from several tens to several hundreds since output voltages of the unit cells configuring the fuel cell are very low. In this instance, a bundle of cells configured by coupling the unit cells in series is referred to as a fuel cell stack.
The unit cells have different operational states in the configuration of cells because of deviation in the manufacturing process, the temperature in the stack, and non-uniform distribution of compression pressures.
When the performance of a single unit cell in the fuel cell stack is degraded, the membrane of the unit cell is damaged because of a high temperature or the electrode thereof is damaged because of a reverse potential, which also occurs in the adjacent unit cells and hence the operation of the fuel cell stack may be substantially influenced by this problem.
Therefore, it is required to use a monitoring device for measuring the normal operation status of the unit cells by monitoring the voltage of each unit cell or the voltages of a group of unit cells.
One of conventional methods for monitoring the fuel cell stack voltage is to measure the voltage states of respective unit cells by using as many analog/digital converters (hereinafter, referred to as ‘A/D converters’) as the number of the stacked unit cells. However, the above-noted method uses a plurality of A/D converters to thus problematically increase the cost.
In the configuration of the fuel cell voltage monitoring apparatus, when the current generated in the stack flows outwardly through a cable such as the ground cable, the currents flowing to the anode and the cathode of the stack become different from each other which may cause a chemical imbalance to the electrodes of the fuel cell. Therefore, it is needed for the fuel cell voltage monitoring apparatus to electrically insulate a control circuit from the output current of the unit cell.
In order to perform electrical insulation and reduce the number of A/D converters, a conventional method switches a plurality of unit cells by using a relay or a multiplexer.
FIG. 1 shows a brief configuration of a conventional fuel cell stack monitoring apparatus using relays and A/D converters.
However, the above-noted prior art generates time differences for detecting the voltages of a plurality of cells, and uses relays for separation so that errors may occur because of degradation of contact performance.
Korean Patent Publication No. 2002-56483, Japanese Patent Publication No. 2004-536437, and U.S. Pat. No. 6,816,797 disclose methods for comparing unit cell voltages with a specific reference voltage and determining voltage greatness to thus simplify signals for monitoring the fuel cell voltage, but they also have a problem of requiring an additional power supply circuit for generating the reference voltage.
The above information disclosed in this Background section is only for enhancement of understanding of the background of the invention and therefore it may contain information that does not form the prior art that is already known in this country to a person of ordinary skill in the art.